Image display devices that express color images by forming dots of each color on a printing medium or on a display medium called a liquid crystal display are widely used as output devices for various types of image equipment. With the concerned image display devices, only a specific limited number of multiple color dots can be formed, but by forming these colored dots mixed at a suitable ratio, it is possible to express color images.
Color image data that is input to an image display device is supplied as data combined with image data of specified colors, and normally, the various colored dots used to express color images by an image display device are used combined with different colors from the specified colors of image data that form the color image data. Because of this, to display color images using an image display device, the following type of data conversion is performed on the color image data, and the various colored dots are formed based on the obtained image data.
First, the color image data expressed as a set of the various colored image data is converted into image data according to specific colors for which it is possible to form dots using an image display device. The process of converting the color image data expressed as a set of specific colors into color image data expressed as a different set of colors in this way is called color conversion from a first color coordinate system into a second color coordinate system, or simply called ‘color conversion’. After a color conversion is done once by an image display device, and color image data are converted by each color into a set of specific colors that can be printed as dots by an image display device, the obtained various colored image data is converted for each color to image data in an expression format based on the dot on-off state. A color image is expressed by the image display device forming various colored dots on the display medium based on the image data in an expression format based on the dot on-off state obtained in this manner.
Color images formed in this manner are expressed by various color dots, so when the dots show clearly, the image has a rough feel, with poor so-called granularity, making for poor image quality. To avoid this kind of degraded granularity, image display devices that can form dots that do not show clearly are widely used. Examples of dots that do not show clearly are small dots or light color dots. If an image is expressed by forming these dots which do not show clearly, it is possible to express images with good granularity and with high image quality.
Also, if these dots that do not show clearly are formed instead of normal dots, the increase portion of the tone value due to forming a certain number of dots, in other words, the tone value that can be expressed per dot, becomes smaller than the normal dots are formed. Therefore, with image display devices that can form small size dots (small dots) or light color dots (light dots), for example, it is possible to express fine level tone changes, and from this perspective as well, it is possible to express high image quality images.
However, although it is possible in theory to express fine level tone change by forming the small dots or the light dots, there are some cases in which fine level tone changes are not expressed enough. Of course, by forming dots that do not show clearly, an image with high image quality for which the dots do not show clearly is expressed, but it is not possible to obtain a sufficient effect of expressing fine level tone changes in addition to expressing images with good granularity. It would be possible to display images of even higher image quality if it were possible to sufficiently draw out the potential performance that image display devices have, making it possible to accurately express fine level tone changes as well.
The present invention was created to solve the problems of the prior art described above, and its goal is to, for an image display device that can form dots that do not show clearly such as small dots or light dots, to provide technology that makes it possible to display images with high image quality by drawing out the natural performance of the image display device.